If he looked exactly the same, people would still be mad. 99% of the reason the "average" western fans don't like the FE characters is because most of them look the same to them, just with different hair colors. Of course, each series has some of this (Mario, Luigi, Wario all look similar), but usually have more variety. Fox, Falco and Cloud can all be considered *somewhat semi clones (not as much Wolf), but all 3 look quite different from each other. Marth, Chrom, Ike, Lucina and now Byleth all look quite similar to the untrained eye (I know that fans will see a huge difference, but besides maybe Ike, I can't really tell the difference). Robin and Corrin actually look pretty different off the top of my head. But a large portion of their series look like the others, just with small differences, like Ike being a little bigger, and Roy having different hair color.
I need to make it clear that I'm NOT salty about Byleth and I'm very happy for his/her fans. But I firmly believe that if he wasn't another blue haired fighter who's main picture shows him with a sword (and the first half of the trailer), he would be more accepted, but only a little. I think many western fans dislike FE more than the others because of how many clones/look alikes take up their ranks, and so even if they get new unique characters, the fans feel negativity towards the series unfortunately.
Not excusing the mentality, but after conversations with my friends (who I feel would be considered average western games) I believe this is a lot of the mentality. "Another blue haired swordsman" (Again, I know he's not a swordsman, but that seems to be what you see when they show him).